Conventional metal halide lamps have a triple tube structure as shown in FIG. 24. In FIG. 24, a metal halide lamp 500 includes a discharge tube 501, an inner tube 502, and an outer tube 503. The discharge tube 501 is housed in the inner tube 502, and the inner tube is housed in the outer tube 503. In such a structure, even if the inner tube 502 is broken by rupture of the discharge tube 501, the broken pieces of the discharge tube 501 and the inner tube 502 remain in the outer tube 503. This prevents the broken pieces of the discharge tube 501 and the inner tube 502 from scattering around.
Assume here that when the discharge tube 501 ruptures, a middle portion 504 of the inner tube 502, which is a portion in the vicinity of the discharge tube 501, gets broken. In this case, an end portion 505 of the inner tube 502 may be blown off toward a closed end portion 506 of the outer tube 503. If the end portion 505 is blown off and collides against the closed end portion 506, the outer tube 503 may be broken and the broken pieces thereof may be scattered around.
In order to prevent such breakage of the outer tube 503, the following structure is proposed in Patent Literature 1. That is, a breakage prevention member 507 for preventing breakage of the outer tube 503 is arranged between the end portion 505 of the inner tube 502 and the closed end portion 506 of the outer tube 503. Since the breakage prevention member 507 prevents the end portion 505 of the inner tube 502 from directly colliding against the closed end portion 506 of the outer tube 503, breakage of the outer tube 503 is prevented.